1. Field of the Invention
The invention relates to an objective. The objective may be configured as a projection objective which may be used in a microlithographic projection exposure apparatus to expose a radiation-sensitive substrate arranged in the region of an image surface of the projection objective with at least one image of pattern of a mask that is arranged in the region of an object surface of the projection objective. The invention also relates to a projection exposure apparatus which includes such objective.
2. Description of the Related Art
Microlithographic projection exposure methods and apparatus are used to fabricate semiconductor components and other finely patterned components. A microlithographic exposure process involves using a mask (reticle) that carries or forms a pattern of a structure to be imaged, for example a line pattern of a layer of a semiconductor component. The pattern is positioned in a projection exposure apparatus between an illumination system and a projection objective in a region of the object surface of the projection objective. Primary radiation from the ultraviolet electromagnetic spectrum (UV radiation) is provided by a primary radiation source and transformed by optical components of the illumination system to produce illumination radiation directed at the pattern of the mask. The radiation modified by the mask and the pattern passes through the projection objective, which forms an image of the pattern in the image surface of the projection objective, where a substrate to be exposed is arranged. The substrate, e.g. a semiconductor wafer, normally carries a radiation-sensitive layer (photoresist).
Various types of primary radiation sources are currently used in the field of microlithography. In some cases, a laser is used as primary radiation source. A natural bandwidth of the laser may be narrowed by appropriate bandwidth narrowing devices. For example, a natural bandwidth of about Δλ=500 pm may be reduced by three orders of magnitude to obtain radiation having a bandwidth Δλ≈0.5 pm used for the exposure. Where radiation with a relatively small bandwidth is used for the exposure, chromatic aberrations caused by the optical elements of the projection objective may be kept relatively small without specific efforts for chromatic correction (correction of chromatic aberrations).
The situation is different in microlithographic systems having primary radiation source emitting ultraviolet radiation from a relatively broad wavelength band. For example, a mercury vapour lamp or a light emitting diode (LED) may be used as primary radiation source. Specifically, projection exposure systems having a central wavelength λ=365.5 nm±2 nm (so-called i-line system) have been in use for a long time. Those systems utilize the i-line of a mercury vapour lamp, the natural bandwidth thereof being limited to a narrower utilized band width Δλ of about 2 nm with the aid of filters or in some other way. Other spectral lines of a mercury vapour lamp may also be used for exposure, such as the mercury h-line at about 405 nm and the mercury g-line at about 436 nm. The electromagnetic spectrum of each of the lines is relatively broad, thereby requiring that the projection objective should provide a relatively efficient correction of chromatic aberrations in order to ensure low-aberration imaging even with such broad band radiation at the required resolution. Chromatic correction is also required if it is desired to use a laser source at the natural line width or a laser source having only moderate line width narrowing.
Chromatic aberrations occur basically because of the fact that the index of refraction, n, of transparent optical materials varies as a function of the wavelength λ. In general, the index of refraction (or refractive index) of transparent optical materials is higher for shorter wavelength than for longer wavelength. Chromatic aberrations may be subdivided into different categories. A first category of chromatic aberrations includes deviations of axial position, shape and/or size of images formed at different wavelength from an ideal image. This accounts for the fact that an image is formed for each wavelength in a paraxial region, and the images usually differ in position and size. The chromatic aberrations of the first category are usually referred to as “axial chromatic aberration” (or axial color, AX) and “chromatic difference of magnification” (or lateral color, LAT).
Axial chromatic aberration is the longitudinal variation of paraxial focus (or image position) with wavelength. Where the index of refraction of an optical material is higher for shorter wavelength than for longer wavelength, the short wavelengths are more strongly refracted at each surface of a lens element so that, in a simple positive lens, for example, the relatively shorter wavelength rays are brought to a focus closer to the lens than the relatively longer wavelength rays. The paraxial distance along the optical axis of the lens between the two focus points is the axial chromatic aberration. When the shorter wavelength rays are brought to focus closer to the positive lens than the longer wavelength rays, the longitudinal axial chromatic aberration is usually termed “undercorrected” or “negative”.
When a lens system forms images of different sizes for different wavelengths, the difference between the paraxial image heights for different colors is usually denoted lateral color or chromatic difference of magnification.
The chromatic variation of the index of refraction also produces a variation of monochromatic aberrations, which may be summarized in a second category of chromatic aberrations. The variation of monochromatic aberrations include the chromatic variation of spherical aberration (also denoted spherochromatism), the chromatic variation of coma, the chromatic variation of astigmatism, the chromatic variation of distortion and the chromatic variation of image field curvature.